Timestamp operation in http_poller input


We are trying to pull timed data from a server with logstash's http_poller input plugin with an interval of one hour.
The server need a header named "start" which should be the time when the request is sent minus one hour (so we do't have recurrent data).

here is our input's config

input { http_poller { urls => { url_source = { method => get url => "http://localhost:8080" headers => { start => "the timestamp should be here" } } } interval => 3600 } }

Is there a way to get the current timestamp and substract one hour so we can add it in the headers ?
Or should we implement some logic on the server side ?

Thanks you very much.